Download Sulfur and carbon isotope analyses of the 2.7 Ga Jeerinah

Geochemical Journal, Vol. 34, pp. 121 to 133, 2000
Sulfur and carbon isotope analyses of the 2.7 Ga Jeerinah Formation,
Fortescue Group, Australia
TAKESHI KAKEGAWA, 1 YUKIO K ASAHARA,1 K EN-ICHIRO H AYASHI1 and HIROSHI OHMOTO2
1
2
Tohoku University, Graduate School of Science, Sendai 980-8578, Japan
The Pennsylvania State University, Astrobiology Research Center, University Park, PA 16802, U.S.A.
(Received January 14, 1998; Accepted October 6, 1999)
Sulfur and carbon isotope ratios have been determined on carbonaceous shales of the 2.7 Ga Jeerinah
Formation, Hamersley Group, Australia. The analyses were performed using the Nd-YAG laser microprobe method.
The δ 13C(PDB) values of organic matter range from –38.3‰ to –35.1‰ with an average of –37.0‰ (44
analyses). These carbon isotope compositions support the previously proposed hypothesis that methanogens
and methanotrophs were involved in the carbon cycles in the 2.7 Ga Hamersley ocean. Sulfur isotope
compositions are ranging from +0.4‰ to +10.2‰ with an average of +4.6‰ (90 analyses). A notable
feature is variable δ 34S values within a micro-scale area; approximately 6 to 7‰ variations of δ34S values
were detected within 5 × 5 mm areas. Such δ 34S variations clearly indicate that pyrites in the Jeerinah
shales were formed as a result of the sulfate reduction. This suggests that the 2.7 Ga Hamersley ocean
contained appreciable amount of dissolved sulfate, opposing to the previously popular H2S-rich ocean
model. Because of no evidence for the hydrothermal alterration on the examined samples, pyrites in the
2.7 Ga Jeerinah shales were most likely formed by the biological sulfate reduction in sediments.
ganisms were active in the 2.7 Ga ocean and that
these organisms played an important role in the
global carbon cycle. These suggestions were based
on the discovery of organic matter with extremely
light δ 13C values, –60 to –40‰, in some sedimentary rocks in the Hamersley Basin in Australia, in
the Abitibi greenstone belt in Canada and the
Transvaal basin in South Africa (Hayes et al.,
1983, 1992; Strauss, 1986; Schidlowski, 1988;
Strauss and Moore, 1992; Strauss and Beukes,
1996). Controversy has also persisted as to
whether sulfate-reducing bacteria were already
active in the 2.7 Ga ocean, because the previously
reported sulfur isotope data on 2.7 Ga sedimentary rocks were ambiguous (Goodwin, 1976;
Lambert and Donnelly, 1992).
The shale samples of the Jeerinah Formation
are characterized by high organic carbon contents
(up to 5 wt %) and no hydrothermal alteration
(Trendall, 1979). Therefore, they are suitable for
an investigation of the ecological system and the
I NTRODUCTION
The carbon isotopic composition of organic
matter in sedimentary rocks provides information
about the nature of ancient organisms and ecological conditions (Schidlowski et al., 1983). For
Archean sedimentary rocks without any fossils,
carbon isotope records of organic matter are the
only clue for the evolution of life during the early
stage of Earth’s history. Sulfur isotopic compositions of pyrite in Archean sedimentary rocks may
provide information if sulfate-reducing bacteria
were active in sedimentary environments
(Raiswell and Berner, 1986).
In order to investigate the sedimentary environments and ecological conditions in oceans 2.7
Ga ago, carbonaceous shales were collected from
the Jeerinah Formation of the Fortescue Group in
the Hamersley district, western Australia. Some
researchers (e.g., Hayes et al., 1983) have suggested that methanogenic and methanotrophic or121
122
T. Kakegawa et al.
Fig. 1. Geological map of the Hamersley-Pilbara district.
activity of sulfate-reducing bacteria in the 2.7 Ga
ocean. In this study, in situ analyses of sulfur and
carbon isotopes were performed on 4 rock specimens of the Jeerinah shales using a Nd-YAG laser microprobe system. The relationship between
sulfate-reducing and methanogenic bacteria in the
2.7 Ga Hamersley ocean will be discussed in this
paper.
G EOLOGY
The Pilbara Craton is mainly comprised of
3.58–2.77 Ga old granite and greenstone. The late
Archean to early Proterozoic sequence, called the
Mt. Bruce Supergroup, overlies the Pilbara Craton
(Fig. 1). The Mt. Bruce Supergroup is subdivided
into three groups based on lithostratigraphy: the
mostly volcanic Fortescue Group, the Hamersley
Group, and the dominantly clastic sedimentary
Turee Creek Group (Trendall, 1979; Arndt et al.,
1991). The Fortescue Group, with a thickness of
150 to 500 m, is comprised of a variety of igneous and sedimentary rocks. The Jeerinah Forma-
Fig. 2. Stratigraphy of the Fortescue Group.
S and C isotope analyses of the 2.7 Ga Jeerinah Formation
Fig. 3. Petrographic feature of pyrite in a Jeerinah
sample.
tion is the uppermost unit of the Fortescue Group
that crops out mostly along the boundary between
the Pilbara block and the Hamersley Basin (Figs.
1 and 2). This formation is predominantly comprised of clastic sedimentary rocks (shale and
sandstone). A U-Pb zircon age of 2687 Ma is reported for a tuffaceous unit of the Jeerinah Formation (Arndt et al., 1991). Many previous investigators (Blake and Barley, 1992; Simonson et al.,
1993) proposed the shallow marine conditions as
a sedimentary environment of the Jeerinah Formation.
SAMPLES
Five shale samples were collected from outcrops ~1 km southeast of the Millstream National
Park (Fig. 1). Two representative samples were
analyzed in detail for the sulfur and carbon isotopic compositions using the laser microprobe
method.
The Jeerinah samples are finely laminated, and
cross laminae were also recognized in some samples. Fine-grained quartz and chlorite (<10 µm)
are the most abundant silicate minerals. Quartz
occurs as rounded-grains, showing equigranular
texture, and chlorite occurs as aggregates of green
fibrous crystals. Epidote and actinolite are also
present as minor silicate minerals. Organic matter is laminated concordantly to the sedimentary
123
bedding. Most pyrite crystals occur in anhedral
shapes with sizes of ~10 to 100 µm (see Fig. 3).
Coarse-grained pyrite crystals (>200 µm in diameter) are only found in later carbonate veins which
cut the sedimentary bedding. Goethite,
pseudomorphic after pyrite, occurs as a minor
mineral in some samples. Samples containing
goethite were avoided for the sulfur isotope analyses. The metamorphic grade of the examined samples is interpreted as lower than greenschist facies,
because the amounts of biotite and muscovite are
small, the sedimentary bedding is well preserved,
and schistosity is absent (Trendall, 1979; Trendall
et al., 1990).
ANALYSES
Sulfur and carbon concentrations of shales
Bulk rock samples were prepared by crushing
rock chips of ~1 cm3 in volume. Approximately
10 mg of powdered samples were used for each
analysis. Sulfur and carbon were analyzed using
a Carlo-Erba Elemental Analyzer. The detection
limit by this analytical method is approximately
0.1 wt % for both sulfur and carbon. The reproducibility of the carbon and sulfur concentrations
is better than 0.2 wt %. Carbonate and sulfate
minerals were not detected through the petrographic and X-ray diffraction studies. This suggests that carbon and sulfur concentrations determined on the Jeerinah samples represent organic
carbon and pyrite sulfur contents.
Sulfur isotopes
Samples used in this study contain micro-sized
pyrite crystals. The primary interest of this study
is to determine the micro-scale variations of δ 34S
values of pyrite and to discuss origins of pyrite. It
is difficult, however, to physically separate each
pyrite. In addition, the conventional Cu 2O combustion method to prepare SO 2 gas for sulfur isotope analyses (Robinson and Kusakabe, 1975) requires a concentrate of approximately 3 mg of
pyrite; this is difficult to achieve if pyrite is disseminated in very fine grains. Because the laser
microprobe method (Kelly and Fallick, 1990;
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T. Kakegawa et al.
Fig. 4. Schematic diagram of the Nd-YAG laser microprobe system at Tohoku University.
Kakegawa, 1993; Kakegawa et al., 1998) can deal
with much smaller amounts of pyrite, we can expect to determine the δ34S values in a micro-scale
area. Therefore, δ 34S values were determined using a laser microprobe system.
The Nd-YAG laser system used at Tohoku
University (Fig. 4) consists of: (a) a Nd-YAG laser microprobe unit, (b) a sample chamber, (c) an
optical system (a binocular microscope and a TV
monitor), and (d) an oxygen flask. The selected
samples were cut into rock chips with a volume
of ~2 cm3 and then placed into the sample chamber. The laser ablation was carried out using a
continuous-wave (C.W.) Nd-YAG laser, under an
O2 atmosphere (8 torr of PO 2 ). The diameter of
the laser beam can be changed by an adjustable
aperture from the multiple mode (~150 µm in laser diameter) and to the TEM00 mode (~30 µm in
laser diameter). Only the TEM 00 mode was
adopted for all laser microprobe analyses at
Tohoku University, because pyrite in the examined samples is smaller than the laser diameter in
the multiple mode.
The Nd-YAG laser was fired on a sample for 5
seconds; the typical output power used in this
study was ~5 watts. The laser ablation typically
created a pit, ~100 µm in diameter, on the surface
of the pyrite sample. The laser beam also
combusted the matrix of rocks as well as the pyrite crystals, when the pyrite grain sizes were
smaller than 100 µm. In addition to SO2 , variable
amounts of H2O, CO2 , and various hydrocarbon
gases are usually generated during laser ablation
of sedimentary rocks. The SO2 was purified before introduction to the mass spectrometer. After
laser ablation, the sample chamber was opened to
the cryogenic traps 1 and 2 (Fig. 4). The H 2O was
condensed in traps 1 and 2, and cooled by an acetone slush (T ≈ –95.4°C). A platinum furnace,
located between traps 1 and 2, was heated to 850°C
to convert hydrocarbons to CO2 and H 2O. Another
cryogenic trap (trap 3), located next to the platinum furnace, condensed SO2 and CO2 gases when
cooled by liquid nitrogen (T = –195°C). After
freezing, the non-condensable gases (mostly O2)
were pumped away. Next, trap 3 was isolated from
trap 2 and the vacuum pump system, and CO2 was
pumped out when trap 3 was cooled by liquid nitrogen-pentane slush (T ≈ –129.7°C).
The purified SO2 gas was transferred from the
cryogenic pentane trap of the purification line to
the cold finger of the micro-inlet line of the mass
spectrometer (Fig. 4) by cooling the cold finger
with liquid nitrogen; the cold finger was then isolated and heated to 40°C. A micro-inlet line was
heated constantly at 40°C to avoid absorption of
SO2 on the metal surface. A Finnigan MAT252
mass spectrometer analyzed the abundance ratio
of masses 66 and 64; these ratios were converted
to the δ34S value. The δ34S values are reported with
respect to the CDT (Canyon Diablo troilite) standard.
S and C isotope analyses of the 2.7 Ga Jeerinah Formation
125
Table 1. S and C concentrations of the
Jeerinah shales
Sample #
Fig. 5. Schematic diagram showing the method of sample preparation and laser ablation for the Jeerinah
shales. Sulfur isotope compositions of each area were
determined on SO2 gas generated from several spots.
Approximately 25 analyses were performed on each
plate.
The reproducibility of the above analyses is
±0.2‰. In general, δ34S values by the laser microprobe tend to be lighter than those by the conventional method by 1 ± 0.2‰ (Kakegawa, 1993).
The δ 34S values shown in the succeeding sections
are those corrected by adding 1‰ to the measured values.
For accurate sulfur isotope analyses using the
laser microprobe, it is necessary to generate more
than 200 nmol of SO2 gas. This corresponds to
the amount of SO 2 generated from a pyrite crystal
of 150 µm in size. However, because the sizes of
most pyrites in the Jeerinah samples are smaller
than the diameter of a laser beam (20 µm), and
the pyrites are sparsely distributed in the shaly
matrix, it was not possible to analyze individual
grains of pyrite.
In order to investigate micro-scale δ 34S variations in both horizontal and vertical directions
(i.e., on and across the bedding planes) in the shale
samples, each sample was cut into a chip with a
volume of ~1 cm3, and sliced into two plates along
the bedding plane (Fig. 5). Different micro-areas
on the surface of each plane were analyzed for
δ 34S. A sufficient amount of SO2 gas for isotopic
analysis was generated from a micro-area of ~0.5
S (wt %)
C (wt %)
S/C
17A
18
19
21
16
0.38
1.11
0.08
0.53
0.20
2.70
2.16
2.26
4.13
2.59
0.14
0.51
0.04
0.13
0.08
average
0.46
2.78
0.18
mm2 by hitting approximately 5 to 20 spots (each
spot is ~20 µm in diameter) by a Nd-YAG laser
under an O2 atmosphere. In this analysis mode,
matrix of sample (i.e., silicate minerals and organic matter) are combusted with pyrite. It is
found that the oxygen isotopes of sample matrix
did not affect on the accuracy or reproducibility
of sulfur isotope data, probably because SO2 gas
is generated simply using O2 gas in the sample
chamber (Kakegawa, 1993). Approximately 25
area analyses were performed on each bedding
plane (laser microprobe area analysis).
Carbon isotopes
During the laser ablation of fine-grained pyrite in carbonaceous shales, a large amount of CO 2
gas is produced from organic matter in addition
to SO 2 gas from pyrite. CO 2 gas is cryogenically
separated from SO2 gas in the standard procedures
of sulfur isotope analyses, and then carbon isotope compositions were determined on separated
CO2 gases. The reproducibility and accuracy of
δ 13C analyses by this method is determined to be
better than ±0.2‰ (1σ) (Liu, 1996). Carbon isotopic ratios are expressed using the δ notation relative to Peedee belemnite standard (PDB). Carbon
(or sulfur) isotope analyses were performed only
when the enough amounts of sample gases (>200
nmol in the cold finger part; Fig. 4) were obtained.
RESULTS
S and C concentrations
Bulk analyses of pyrite sulfur (Spy) and organic
carbon (Corg) concentrations were performed on 5
Table 2. δ 34S values of pyrite in the Jeerinah shales determined by the laser microprobe method
126
T. Kakegawa et al.
S and C isotope analyses of the 2.7 Ga Jeerinah Formation
rock specimens. The results show that the Corg
concentration ranges from 2.2 wt % to 4.1 wt %
with an average of 2.8 wt %, and Spy concentration ranges from 0.1 wt % to 1.1 wt % with an
average of 0.5 wt % (Table 1). The S/C ratios range
from 0.04 to 0.51 with an average of 0.18.
127
Sulfur isotopic compositions
Laser microprobe analyses were performed on
a total of 90 micro-areas in four rock specimens
cut from two samples (Aus 18 and Aus 21; Table
2). Approximately 20 analyses were completed on
two different sedimentary bedding planes in each
sample, separated by 0.5 cm in stratigraphy. The
results of the laser microprobe analyses are summarized in Fig. 6.
δ 34S values on each sedimentary bedding vary
by 6 to 7‰. The two different planes of sample
Aus 18 (18-2 and 18-3 in Fig. 6) show similar
variations in δ 34S values, ranging from 0.4‰ to
7.4‰ with an average of 3.8‰. The δ 34 S frequency patterns of each plane are also similar
(Figs. 6(a) and (b)). The two planes of sample Aus
21 (21-1 and 21-2) show slightly different fre-
Table 3. δ 13C of organic matter in the Jeerinah shales
determined by laser microprobe analysis
Fig. 6. δ34S variations for disseminated, fine-grained
pyrite in the Jeerinah shales determined by the laser
microprobe analysis. (a) First plane of sample Aus 18;
(b) second plane of sample Aus 18; (c) first plane of
sample Aus 21; (d) second plane of sample Aus 21;
(e) total range.
Sample I.D.
δ1 3 C (‰)
Aus18
Aus18-2-2-a
Aus18-2-2-b
Aus18-2-2-c
Aus18-2-2-d
Aus18-2-2-e
Aus18-2-2-f
Aus18-4-2-a
Aus18-4-2-b
Aus18-4-2-c
Aus18-4-3-a
Aus18-4-3-b
Aus18-4-3-c
Aus18-4-3-d
Aus18-4-3-e
Aus18-4-3-f
Aus18-4-3-g
Aus18-4-3-h
–35.1
–35.7
–35.7
–36.0
–36.0
–35.7
–36.3
–36.5
–36.4
–36.3
–36.2
–36.2
–35.9
–35.4
–36.0
–36.3
–36.3
average
–36.0
Sample I.D.
δ1 3 C (‰)
Aus21
Aus21-0-a
Aus21-0-b
Aus21-0-c
Aus21-0-d
Aus21-0-e
Aus21-0-f
Aus21-0-g
Aus21-0-h
Aus21-0-i
Aus21-0-j
Aus21-0-k
Aus21-1-1-a
Aus21-1-1-b
Aus21-1-1-c
Aus21-1-1-d
Aus21-1-1-e
Aus21-1-1-f
Aus21-1-1-g
Aus21-1-2-a
Aus21-1-2-b
Aus21-1-2-c
Aus21-1-2-d
Aus21-2-1
Aus21-3-2-a
Aus21-3-2-b
Aus21-3-2-c
Aus21-3-2-d
–37.7
–37.6
–38.1
–37.7
–38.1
–37.7
–37.3
–37.5
–37.2
–37.5
–37.3
–37.2
–37.9
–38.0
–37.6
–38.1
–38.1
–36.7
–37.3
–37.8
–38.2
–37.2
–37.4
–37.9
–38.4
–38.0
–38.3
average
–37.7
128
T. Kakegawa et al.
Table 4. δ34S values of pyrite and δ13C values
of organic matter in the Jeerinah shales determined by the conventional method
Sample I.D.
Aus18
(1)
(2)
(3)
(4)
(5)
Aus21
(1)
(2)
(3)
(4)
δ3 4 C
(‰)
+3.8
+4.8
+5.2
+5.2
+5.3
+5.1
+5.3
+6.0
+4.1
Sample I.D.
Aus18
(1)
(2)
(3)
(4)
(5)
Aus21
(1)
(2)
(3)
δ1 3 C
(‰)
–36.9
–36.8
–37.0
–37.1
–37.2
–38.9
–39.0
–38.9
–37.7‰ (Fig. 7). These results show that δ13 C values are relatively constant in micro-areas, compared to the δ 34S variations (Fig. 6). Previous investigators reported δ 13Corg values, ranging from
–48.6‰ to –27.9‰ with an average of –39.6‰
for 12 analyses of the Jeerinah Formation (Fig.
7(c); Schidlowski et al., 1983; Strauss and Moore,
1992). The new set of δ 13C values determined in
this study fall in the range of previously reported
values.
Fig. 7. Carbon isotope variations of organic matter in
the Jeerinah shales. δ 13C values were determined by
the laser microprobe analysis. (a) Sample Aus 18;
(b) sample Aus 21; (c) previously published data
(Schidlowski et al., 1983; Strauss and Moore, 1992).
quency patterns and ranges of δ 34S values: from
3.5‰ to 10.2‰ with an average of 6.0‰ for the
first plane, and from 2.2‰ to 8.8‰ with an average of 5.4‰ for the second plane (Figs. 6(c) and
(d)).
Bulk rock analyses of sulfur and carbon isotope
compositions
Results of bulk rock analyses of sulfur and
carbon isotope compositions by the conventional
method are listed in Table 4. Sulfur isotope compositions are ranging from +3.8‰ to +5.2‰ for
sample Aus 18 and +4.1‰ to +5.3‰ for sample
Aus 21. Carbon isotope compositions are ranging
from –37.2‰ to –36.8‰ for sample Aus 18 and
–39.0‰ to –38.9‰ for sample Aus 21.
DISCUSSION
Carbon isotopic compositions
Laser microprobe analyses were performed on
a total of 44 micro-areas in two samples (Aus 18
and Aus 21; Table 3). The δ13C values of organic
matter range from –36.5‰ to –35.1‰ in sample
Aus 18 with an average of –36.0‰, and –38.4‰
to –36.7‰ in sample Aus 21 with an average of
Significance of carbon isotope compositions
Extremely light carbon isotopic compositions,
such as a δ 13C value of –48.6‰, have been reported by Schidlowski et al. (1983) and Strauss
and Moore (1992) for the Jeerinah Formation (Fig.
7(c)). These light δ 13C values have previously
S and C isotope analyses of the 2.7 Ga Jeerinah Formation
129
Fig. 8. Cartoons comparing three models in terms of chemistry and ecological conditions of the 2.7 Ga Hamersley
ocean. Symbols A, T, F and S represent oxygenic photoautotrophs, methanotrophs, methanogens and sulfate reducers. (A) is the model proposed by Hayes (1994) for the 2.7 Ga Hamersley ocean. (B) and (C) are the proposed
models in this study.
been interpreted to suggest that methanogens and
methanotrophs were involved in carbon cycles in
the surface environments of the early Earth (Hayes
et al., 1983; Hayes, 1994). In this model,
methanogens produce CH 4 in deep oceans, and the
methanotrophs oxidize CH4 to 13C-depleted CO2
using O2 produced by photosynthetic bacteria in
the upper part of the oceans (Fig. 8(A); Hayes,
1994). Incorporation of such 13 C-depleted CO2 (or
CH 4 ) into biological activity produces organic
matter with extremely light δ 13C values (<–35‰).
Many previous investigators suggested that
diagenetic or metamorphic degradation of organic
matter causes the shift of δ 13C org compositions
towards heavier values, and the shift has been related to the H/C ratio of the kerogen (Hayes et
al., 1983; Schidlowski, 1988; Watanabe et al.,
1997). The disadvantage of carbon isotope analysis using the laser microprobe is that the H/C ratios of kerogen cannot be determined on the same
point where the δ 13Corg values are determined.
Although the absence of H/C ratios of the examined samples makes the interpretation of the measured δ13C values difficult, the original δ13C values could be lighter than the measured δ13C values (–38 to –35‰). If this is the case, the δ 13C
values of this study give more supportive data for
the hypothesis that methane was incorporated in
biological activity in the Hamersley ocean at 2.7
Ga.
Origin of pyrite in the Jeerinah Formation and
activity of sulfate-reducing bacteria
The primary object of this study is to examine
if sulfate was abundant and sulfate-reducing bacteria were active in the 2.7 Ga Hamersley Basin.
Sulfate-reducing bacteria are able to form Fesulfides by secreting H 2S as a product of the reduction of dissolved sulfate. In general, the Fesulfides form by reaction between H2S and Fehydroxides (Berner, 1984). The reduction of
sulfate is accompanied by a kinetic isotope effect
(∆ sulfate-sulfide) in which H 2S produced by sulfate
reduction is enriched in 32S with respect to the
substrate SO 42–.
The environmental systems for pyrite formation by sulfate-reducing bacteria are mainly divided into: (1) a closed-bottom system with respect to sulfate supply, where the rate of reduction is faster than the rate of sulfate supply; and
(2) an open-bottom system with respect to sulfate
supply, where the rate of sulfate supply is greater
than the rate of sulfate reduction (Schwarcz and
Burnie, 1973; Ohmoto et al., 1991; Ohmoto,
1992).
The closed-bottom system is represented by
interstitial water in a wet sediment column where
the diffusive supply of seawater sulfate is limited.
Sulfate reduction in a system closed with respect
to sulfate supply is analogous to the Rayleigh distillation process. At the top of the sediment-
130
T. Kakegawa et al.
seawater interface, δ 34S value of pyrite becomes
lighter than the δ 34S value of seawater sulfate by
the ∆ value. The δ 34S value of sulfate becomes
continuously heavier with depth, because the
lighter isotope (32S) is preferentially removed to
form pyrite. Correspondingly, δ34S value of pyrite in the deeper part of a closed-bottom system
becomes isotopically heavier and the total range
of δ34S values of pyrite grains formed in the closed
system becomes variable.
δ34S values of pyrite formed in an open-bottom system (syngenetic pyrite) may have different compositions compared to pyrite formed in a
closed-bottom system (diagenetic pyrite). In an
open-bottom system, the kinetic isotope effect,
∆sulfate-sulfide, would essentially remain constant at
different parts of an open-bottom system and also
among different systems if the nature of organic
matter and temperature are essentially the same
(Ohmoto, 1992). The sulfides formed in such a
system should show uniform δ 34S values that deviate from the seawater sulfate values by the kinetic isotope effect. An anoxic water-column in
an euxinic basin, such as the Black Sea and the
Baltic Sea, is a typical example of the open-bottom system (e.g., Sweeney and Kaplan, 1980;
Leventhal, 1983; Boesen and Postma, 1988). Homogeneous sulfur isotope compositions of pyrite
formed in water-column of the Black Sea indicate
that bacterial sulfate reduction in the open-bottom system results in homogeneous sulfur isotope
compositions of pyrite (Lyons, 1997). Therefore,
it is possible to constrain if pyrite was formed in
anoxic water column or within sediments by examining the heterogeneity of sulfur isotope compositions.
Highly variable δ 34S values are found in micro-scale areas of the four Jeerinah samples (Fig.
6); approximately 6 to 7‰ variations are found
on each small area (5 × 5 mm). These variable
δ34S values within a micro-scale area are considered to be the evidence of sulfate reduction in
sediments during early diagenesis. Dissolved
sulfate may have been reduced to sulfide either
by an abiological or biological process (Trudinger
and Chamber, 1985; Goldhaber and Orr, 1995;
Ohmoto and Goldhaber, 1997). Because the
Jeerinah samples preserved original sedimentary
textures and do not show any features of hydrothermal alteration, abiological (thermochemical)
sulfate reduction is unlikely to explain the sulfur
isotope heterogeneity shown in Fig. 6. Therefore,
the sulfate-reducing bacteria were responsible for
pyrite formation in the Jeerinah shales. This postulates that the 2.7 Ga Hamersley ocean contained
an appreciable amount of dissolved sulfate (>1/3
of the present ocean value) to be reduced (Ohmoto
and Felder, 1987).
It is known that sulfur concentrations in modern fresh water sediments are extremely low (<0.1
wt %; Berner, 1984), because of low concentrations of dissolved sulfate in fresh water. Such a
fresh water environment is a favored model for
some researchers as a depositional environment
of the Jeerinah Formation (Fig. 8(A); Hayes,
1994). However, the fresh water or lacustrine environment model cannot explain the moderate
amount of sulfur concentrations of Jeerinah samples (Fig. 9). Therefore, a marine environment is
more likely for the depositional environment as
proposed by previous investigators (e.g., Blake
and Barley, 1992). The results obtained through
this study oppose to the previously popular theory
that the Archean oceans were sulfate-poor and
most pyrites in Archean sediments are products
of inorganic reaction between dissolved Fe2+ and
H2S in oceans (Canfield and Teske, 1996).
Fig. 9. Correlation between pyrite sulfur and organic
carbon of the Jeerinah shales. A regression line of
modern sediments are also shown (solid line) for comparison
S and C isotope analyses of the 2.7 Ga Jeerinah Formation
131
Strauss (1986) porposed that the δ 34S value of
the 2.7 Ga ocean was approximately +5‰. Because the lightest δ 34S value among the examined
samples is approximately 0‰, the minimum kinetic isotope effect (∆ sulfate-sulfide) associated with
microbial sulfate reduction is, therefore, estimated
to be ~5‰ using the seawater value by Strauss
(1986; Fig. 6). The kinetic isotope effects of modern sediments often exceed 45‰ (Canfield and
Teske, 1996). On the other hand, the kinetic isotope effects found in the Jeerinah samples and
other Archean samples are much smaller than these
of modern or Phanerozoic sediments (Ohmoto et
al., 1993). The magnitude of the kinetic isotope
effect associated with microbial sulfate reduction
will depend on environmental factors such as temperature, types of hydrogen donor (sugar, organic
acid, molecular hydrogen), and etc. (Kemp and
Thode, 1968; Ohmoto and Felder, 1987). In general, the surface temperature of the Archean Earth
is considered to have been higher than that of the
present Earth due to the high PCO 2 level of the
atmosphere (Kasting, 1993). Nature of organic
matter in Archean sediments could have been more
digestible for sulfate-reducing bacteria, compared
to the modern sediments (Hayes et al., 1983).
These factors may cause the high activity of
sulfate-reducing bacteria, resulting in the small
kinetic isotope effect in the 2.7 Ga Jeerinah shales.
sediments, and they are not coexisting as a mixed
culture. This is because sulfate reducers
outcompete methanogens in sulfate-rich environments (Lovely and Klug, 1983). Heterogeneous
sulfur isotope compositions of Jeerinah shales
suggest that sulfate-reducing bacteria were probably restricted in sediments rather than in watercolumn. If this is the case, two models can be proposed for the relationship between methanogenic
and sulfate-reducing bacteria. One is the case (case
(B) in Fig. 8) that only methanogenic bacteria were
active in the anoxic parts of oceans, although
sulfate-reducing bacteria were active in sediments.
Such ecological relationship is the reversed relationship compared to the modern environments.
The other is the case that both anaerobic bacteria were restricted in sediments ((C) in Fig. 8).
This case is very similar to the Phanerozoic conditions, although a large uncertainty exists as to
weather such a modern style of ecological relationship was already established by 2.7 Ga. If (C)
is the case, the activity of methanogens in
sediments had to be rather high in 2.7 Ga
sediments, in order to produce large amount of
CH4 to be oxidized by methanotrophs. It is still
uncertain which model is correct or if the other
possibility exists.
Relationship between sulfate-reducing and
methnogenic bacteria in the 2.7 Ga Hamersley
ocean
It is discussed in the previous section that extremely light carbon isotope compositions were
caused by involvement of methanogenic and
methanotrophic bacteria in the carbon cycle in 2.7
Ga oceans (Fig. 7). Heterogeneous sulfur isotope
compositions of pyrite are most likely caused by
an action of sulfate-reducing bacteria (Fig. 6).
Aerobic methanotrophic bacteria were probably
living in oxygenated parts of 2.7 Ga oceans, separated from anaerobic methanogenic and sulfatereducing bacteria. In modern marine sediments,
both methanogenic and sulfate-reducing bacteria
are restricted to live in anoxic parts of oceans or
(1) δ13C values of disseminated organic matter in sedimentary rocks of the Jeerinah samples
range from –38.4‰ to –35.1‰ (PDB). The 13Cdepleted δ 13C values were probably caused by involvement of methane in the biological carbon
cycle in the 2.7 Ga Hamersley ocean.
(2) Highly variable δ 34 S values (+0.4‰ to
+10.2‰) were found in micro-scale areas of the
Jeerinah shales. This suggests that (a) the 2.7 Ga
Hamersley ocean was sulfate-rich and (b) pyrite
in the Jeerinah samples was formed by reduction
of seawater sulfate in sediments rather than in the
water-column. Sulfate-reducing bacteria were responsible for sulfide formation in Jeerinah shales.
(3) The kinetic isotope effect (∆ sulfate-sulfide)
associated with the bacterial sulfate-reduction was
CONCLUSIONS
132
T. Kakegawa et al.
5‰, much smaller than these of Phanerozoic
sediments.
(4) Methanotrophic, methanogenic and sulfatereducing bacteria were active in the 2.7 Ga
Hamersley ocean. However, they were not coexisting as mixed cultures.
Acknowledgments—Authors wish to thank M. Arthur,
D. H. Eggler, D. P. Gold, A. W. Rose, K. Osseo-Asare
and K. Yamaguchi at The Pennsylvania State University and S. R. Poulson at University of Wyoming for
their suggestions to improve this manuscript. This study
was supported by the Ministry of Education, Science,
Sports and Culture, Japan under Grant No. 03102002
and No. 07041081 to H.O.
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